Contemporary methods for treating patients infected with bacteria involve selecting antibiotics for therapy. Most conventional therapy is based on the physician's experience. In simplistic terms, the physician simply chooses a broad spectrum antibiotic thought to be effective against the most common infectious agents associated with the patient's symptoms. In some instances, however, selection of the appropriate therapeutic is based on susceptibility testing. Susceptibility testing guides the medical practitioner by defining which therapeutics will have the highest probability of successfully treating a particular pathogen, and which antimicrobial agents will most probably be impotent. Susceptibility testing is an important tool, especially when a patient is identified as having a mycobacterial infection, and most especially when the patient presents with tuberculosis (TB: caused by Mycobacterium tuberculosis complex bacteria (MTB)). Current federal recommendations include susceptibility testing on all new cases of TB (Tenover, F. C. et al., Jour. Clin. Micro. 31:767–770 (1993)). Susceptibility testing is an invaluable tool that virtually all physicians rely on at one time or another to select the appropriate antibiotic therapy for their patients.
Methods of treating bacterial infections are constrained by the spectrum of activity of a particular compound. For example, not all bacteria are susceptible to a given antimicrobial compound. Different classes of bacteria are resistant to the action of the different classes of antibiotics. Generally speaking the spectrum of activity of a given antibiotic falls into discrete groups with respect to the class of organisms it affects (e.g., mycologic vs. bacterial, or gram positive bacteria vs. gram negative bacteria).
Antibiotics exert their effect by interfering with a variety of cellular functions. For example, different classes of antibiotics are known to interfere with various aspects of cell wall synthesis, RNA/DNA synthesis, DNA replication, or protein synthesis. Bacteria are resistant to different antibiotics for a variety of reasons. Quintiliani, R. et al., In: Murray, P. R. et al., eds. Manual of Clinical Microbiology, ASM Press, Washington, D.C. (1995) pp. 1308–1326 review several of these resistence mechanisms and point out that the antibiotic must first enter the cell, and only then can it exert its effect at the site of action. The basis for resistence can either be due to the permeability of the organism, or the molecular configuration of the site of action might either be incompatible or nonexistent. In addition, the bacteria might modify, destroy or eliminate the agent.
Resistance can either be innate or acquired. Acquiring resistance can be either by acquisition of genetic material (e.g., transposable elements), or by the inherent infidelity of DNA replication (e.g., point mutations). Mycobacteria appear to have an additional mechanism of resistance. Heifets, L. B. In: Drug Susceptibility in the Chemotherapy of Mycobacterial Infections, Heifets, L. B. ed. CRC Press, Boston, Mass. (1991), pp. 13–57 classifies subpopulations of infecting MTB cells as either actively growing, or in different stages of dormancy. Dormancy permits these subpopulations to survive during patient therapy.
Treating mycobacterial infections is further complicated by the complex pattern of susceptibility. For example, while MTB infections can usually be treated effectively with isoniazide (INH) and/or pyrazinamide (PZA), MAC isolates are typically resistant to these drugs, and M. fortuitum and M. chelonae isolates are usually resistant to all the front line antituberculosis agents.
Tuberculosis is the most prevalent infectious disease in the world today, infecting approximately one-third of the world's population, some 1.7 billion people (Kochi, A. Tubercle 72:1–6 (1991)). In addition, tuberculosis kills more people worldwide (approximately 3 million annually) than any other single infectious disease (Morbidity and Mortality Weekly Report 42:961–964 (1993)). The vast majority of TB cases are in developing countries, however, multi-drug resistant (MDR) strains of MTB (MDR-TB) have become a significant problem globally (World Health Organization (document WHO/TB/96.198) Groups at Risk. WHO Report on the Tuberculosis Epidemic 1996 (1996)). Unless something is done to stem the rise in MDR-TB, a return of the past where tuberculosis was the most common cause of death in both developing and industrialized countries is inevitable.
Other mycobacteria such as Mycobacterium avium complex (MAC), M. paratuberculosis, M. ulcerans, M. leprae, M. kansasii, and M. fortuitum complex are common pathogens as well (see: Wayne, L. G. Clin. Micro. Rev. 5:1–25 (1992) or Falkinham, J. O. Clin. Micro. Rev. 9:177–215 (199) for reviews of the different mycobacterial pathogens). MAC causes disseminated disease in almost half of all late stage AIDS patients (Nightingale, S. D. et al., Jour. Infect. Dis. 165:1082–1085 (1992)). The World Health Organization estimates that by the year 2000 the number of people infected with human immunodeficiency virus (HIV) could exceed 40 million (World Health Organization (document WHO/GPA/CNP/EVA/93.1) Global Programme on AIDS (1993)). M. paratuberculosis (a subspecies of M. avium) causes Johne's disease in ruminants. It has been estimated that Johne's disease costs the U.S. farming industries (e.g., diary and beef) in excess of $1.5 billion annually due to lower productivity and fecundity (Whitlock, R. Proceedings of the Third International Colloquium on Paratuberculosis, pp. 514–522 (1991); Whitlock, R. et al, Proceedings of the 89th Annual Meeting of the United States Animal Health Association, pp. 484–490 (1985)). Mycobacterial diseases extract an enormous social cost.
The most common and best characterized class of antibiotic compounds is by far the β-lactams. Due to the depth and breadth of these antibiotics, the ability to treat mycobacterial infections with these agents would provide significant advantages. Application of the β-lactams in therapeutic regimes designed to treat mycobacterial infections has been tried with limited success (Chambers, H. F. et al., Antimicrob. Agents Chemo. 39:2620–2624 (1995)). The ability to broaden the susceptibility of the mycobacteria, especially to the β-lactams by addressing resistance mechanisms, has significant potential in effectively treating mycobacterial infections.
The invention described herein outlines novel methods and compositions wherein the susceptibility of the mycobacteria can be characterized. These methods and compositions alter the susceptibility of these bacteria to enhance the effectiveness of antibiotics, especially the β-lactam antibiotics. Such methods and compositions will permit the same to be used as part of an effective therapy for defining and/or treating such infections.